1. Field of the Invention
The present invention relates to a charged particle beam irradiation system and charged particle beam extraction method. More particularly, the present invention relates to a charged particle beam irradiation system and a charged particle beam extraction method that are ideally adaptable to a particle therapy system that treats a cancer by irradiating an affected part (an object to be irradiated) with a charged particle beam (ion beam) such as a proton or a heavy ion.
2. Description of the Related Art
A particle therapy method is known as a radiation therapy for cancers, in which a cancer affected part of a patient is irradiated with an ion beam such as, for example, a proton or heavy ion beam. A particle therapy system used in such a therapy may include an ion beam generator, a beam transport line, and an irradiation apparatus. The ion beam generator includes a synchrotron or a cyclotron that accelerates the ion beam that orbits along a circular path and thereby increases its energy to a required level.
The synchrotron includes a radiofrequency acceleration system (acceleration cavity), an extraction radiofrequency electrode, and an extraction deflector (see, for example, JP 2596292 B2). Specifically, the radiofrequency acceleration system accelerates the ion beam that orbits along a circular path to a target level of energy by feeding a radiofrequency voltage to the ion beam. The extraction radiofrequency electrode augments a betatron oscillation amplitude of the orbiting ion beam. The extraction deflector takes the ion beam out of the circular path. When the ion beam which has been accelerated to the target level of energy is to be extracted from the synchrotron to the beam transport line, a radiofrequency magnetic field or a radiofrequency electric field (hereinafter referred to as a radiofrequency signal) is fed to the extraction radiofrequency electrode to thereby augment the betatron oscillation amplitude that is the natural oscillation of the orbiting ion beam. The ion beam with the augmented betatron oscillation amplitude moves out of a stability limit, and is extracted from the synchrotron to the beam transport line and transported onto the irradiation apparatus.
The irradiation apparatus shapes the ion beam, which is introduced from the ion beam generator, in accordance with the depth of the affected part from a patient's body surface and the shape of the affected part and irradiates the affected part of the patient on a treatment couch with the shaped ion beam. The irradiation apparatus irradiates the affected part with the ion beam by using an appropriate beam irradiation method. In general, the irradiation apparatus uses a double scatter method (see page 2081, FIG. 35 of non-patent document 1, Review of Scientific Instruments, vol. 64, no. 8 (August 1993), pages 2074 to 2093), RMW irradiation method (see page 2077, FIG. 30 of non-patent document 1), wobbler method (see page 2084, FIG. 41 of non-patent document 1), or beam scanning method (see pages 2086 to 2090 of non-patent document 1 and page 197 of non-patent document 2, Nuclear Instruments and Method in Physics Research Section A, 522 (2004), pages 196 to 204). A raster beam scanning method (see pages 2087 to 2089 of non-patent document 1) is available as a type of the beam scanning method.
The affected part usually has a certain thickness in the direction of the ion beam traveling in a patient body. To irradiate the entire thickness of the affected part with the ion beam, the energy of the ion beam must be controlled so as to form a uniform absorbed dose range (spread-out Bragg peak; hereinafter abbreviated to SOBP) having a certain width in the ion beam traveling direction. A scattering irradiation method that uses a range modulation wheel (hereinafter referred to as the RMW) is proposed as an energy control means for forming a desired SOBP. The RMW is a rotating structure having a plurality of wedge-shaped energy absorbers disposed in a circumferential direction so that the thickness of a region through which the ion beam passes varies with time. The RMW is arranged such that the thickness in the ion beam traveling direction (the axial direction of the RMW) increases or decreases as the RMW rotates. Such an irradiation method as that which uses the RMW is called an RMW irradiation method.
The irradiation apparatus shapes the ion beam in accordance with the depth from the patient's body surface and the shape of the affected part. The current intensity of the beam incident on the affected part is, however, adjusted by the ion beam generator. In the synchrotron, the beam current intensity of the ion beam extracted from the ion beam generator is controlled by adjusting the intensity of the radiofrequency signal (amplitude of the radiofrequency voltage) to be fed to the extraction radiofrequency electrode (non-patent document 2). The cyclotron, on the other hand, includes a device that adjusts the intensity of an extracted ion beam (see, for instance, JP 2004-529483 A). More specifically, the cyclotron measures the intensity of an actually extracted beam and controls an arc current that is to be supplied to an ion source based on the measurements.
The synchrotron receives the injected ion beam from a preaccelerator, accelerates the injected ion beam to a desired level of energy, and extracts the accelerated ion beam. The synchrotron repeatedly performs one cycle of operations that includes injecting an ion beam, accelerating the injected ion beam, and extracting the accelerated ion beam. Unlike the supply of ion beams to the cyclotron, therefore, the ion beam is supplied to the synchrotron only at the time of injecting the ion beam within one operating cycle. The amount of accumulated charge of ion beams accelerated by the synchrotron is maximized at the end of acceleration and decreases with the lapse of the extraction control time (non-patent document 1). Further, it is known that the relationship between the amplitude of the radiofrequency signal to be fed to the extraction radiofrequency electrode and the beam current intensity extracted from the synchrotron is also affected by the accumulated beam charge amount within the synchrotron. In non-patent document 3, Medical Physics, vol. 34, no. 3, March 2007, pages 1085 to 1097, therefore, an amplitude modulation waveform of an extraction radiofrequency signal required for making constant a change with time in the beam current intensity is provided based on an estimation made of the decrease in the accumulated beam charge amount in the synchrotron as the beam extraction control is performed. To minimize effect from fluctuations in the accumulated beam charge amount in the synchrotron, the accumulated beam charge amount in the synchrotron is measured with a DC current transformer (DCCT) and, in accordance with the measurement, a target intensity of the extraction beam current is set. In addition, to suppress a ripple component produced in the extraction beam, feedback control is applied relative to the amplitude modulation waveform of the extraction radiofrequency signal based on the current intensity of the extraction beam observed on a beam ripple monitor. Non-patent document 3 also proposes, in the beam scanning irradiation method, to set a required value of the extracted beam current intensity corresponding to a beam extraction gate width which is defined from the average breathing cycle of the patient measured in advance.
In the RMW irradiation method, on the other hand, a technique is known (JP 2006-239404 A), by which desired SOBP formation can be achieved by controlling the thickness of a periodic structure of the RMW through which the beam passes in order to allow a single RMW to respond to a plurality of patients. Specifically, the thickness of the RMW through which the beam passes is controlled by performing ON/OFF control of the extraction beam in accordance with the thickness of the periodic structure of the RMW adapted to the desired SOBP formation. When the synchrotron is applied to the ion beam generator of the charged particle beam irradiation system, the ON/OFF control of the extraction beam can be achieved by performing the ON/OFF control of the radiofrequency signal fed to the extraction radiofrequency electrode.